Abstract: Some equipments insulation have low points because of the operation environment and service life with the developing of power system. This paper analyzes one differential protection action caused by low voltage insulation of main transformer decreasing. The B phase ground fault of 129 line in 10 kV Ⅱ bus caused the A, C phrases voltage increasing. The insulation between the back of 102 breaker and the main transformer decreasing caused the three phase short circuit. Then the 2# main transformer differential protection acted. This confirmed that the insulation of the connecting and cutting row back of 102 breaker was weak. The main transformer ground point is not suitable in the connecting and cutting row back of breakers.